Construction vehicles, such as large dump trucks that operate at quarries and/or large-scale construction sites, operate for a long time while carrying a heavy load. Large heavy duty tires mounted on such a construction vehicle are required not only to have excellent cut resistance but also to prevent tire failure by suppressing heat build-up which is achieved by suppressing overheating of the tire. Cut resistance is a property that makes it difficult to damage a tire when the tire is brought into contact or collision with a barrier or external object. Heat build-up is a property that mitigates the impact by heating up the rubber by converting the energy applied to the tire via such a physical impact into heat. Because of this, in order to make the cut resistance of the tire to be excellent, large heat build-up of the rubber is required. On the other hand, in order to avoid overheating of the tire and failure due to the overheating, heat build-up of the rubber is required to be low. Thus, the cut resistance and low heat build-up are in a trade-off relationship.
Since the pneumatic tire for construction vehicles described above has a significantly large tire size, molding of the tire is very difficult. For example, when a tire tread portion of such a large tire is molded, it is not possible to mold a green tire by integrally extrusion-molding the rubber portion which forms the tread portion and winding the extruded rubber portion in the same manner as for small tires. Because of this, typically, a tread portion of a large tire is formed by laminating a plurality of rubber sheets having a smaller thickness than the thickness of the designed tread portion, or by spirally winding a rubber strip in a manner that the wound rubber strip overlaps itself. In forming methods that laminate rubber sheets or that wind a rubber strip around in a manner that the wound rubber strip overlaps itself, it is necessary to compound a softening agent or the like in the rubber composition that constitutes the rubber sheet or rubber strip, in order to ensure suitable moldability. However, since compounding agents such as softening agents make heat build-up of rubber compositions greater, it has been difficult to achieve both moldability and low heat build-up in large tires.
International Patent Publication No. WO/2010/077232 proposes to compound silica, carbon black, a silane coupling agent, sulfur, and a sulfenamide accelerator at specific proportions into natural rubber in order to reduce the rolling resistance of tires for large vehicles. However, with this rubber composition, effects of enhancing cut resistance and moldability are not always sufficient. Because of this, further enhancement that can achieve both low heat build-up, and cut resistance and moldability has been demanded.